The present invention relates to a chemical heat pump.
The principle of the operation of the chemical heat pump is well-known, see for example U.S. Pat. Nos. 5,440,889, 5,056,591, 4,993,239, 4,754,805 and the published International patent application WO 94/21973. In a chemical heat pump an active substance, the absorbent, is used which performs the very process of the heat pump and which works together with a volatile medium, the absorbate or sorbate, which usually is a dipolar liquid, in most cases water. As the working active substance, according to the known technology, either a solid substance or a liquid substance can be used. A solid substance has the advantages that the vapor pressure remains constant during the whole discharging process for a constant cooling temperature and a relatively large capacity of storing heat. A typical value of the storage capacity for a solid substance having water as the sorbate, counted as cooling energy, is about 0.3 kWh/1 substance. A further advantage associated with a solid substance is that no movable components are required in the system. Heat is transferred to or conducted away from the substance through a lamellar heat exchanger or a plate heat exchanger in a homogenous contact with the substance. The disadvantage associated with a solid substance is the limited power which can be obtained, due to the bad thermal conductivity of solid substances. For systems the charging time of which corresponds to for example six hours of daytime charging using solar energy and the discharging time of which corresponds to a period of twelve hours of cooling for example a building, this provides no major problem. However, a disadvantage is that for continuous cooling at day and night and based on solar energy two installations working in parallel with each other are required.
A liquid substance has the advantage of a high power since the substance can be spread over the heat exchanger both in charging and in discharging and thereby be efficiently cooled or heated respectively. The disadvantage of a liquid substance is that the cooling capacity decreases as a function of the dilution of the sorbate. This actually strongly limits the operational interval within which the substance can be used, what in turn reduces the storage capacity, counted as above as cooling energy per liter substance. Most liquid substances or absorbent used in chemical heat pumps comprise solutions of strongly hygroscopical inorganic salts in preferably water together with which water is used as the volatile liquid, the sorbate. Then another limitation is obtained by the fact that the dissolved substance cannot be allowed to crystallize. Crystallization creates problems in spray nozzles and pumps. Thus, the use of a liquid substance is limited to converting heat energy to cooling without any storing of heat and systems therefor are generally known and utilized. In such a process e.g. a lithium bromide solution can be used which when heated is evaporated to pass from a diluted solution to a more concentrated solution. This can be made in a chemical heat pump at a low pressure or at atmospheric pressure using air flows. The amount of working substance is relatively small, since no storing of xe2x80x9cchargedxe2x80x9d concentrated solution is made. The hot concentrated solution is then cooled and is then again made to absorb the sorbate which is evaporated from a heat exchanger, the heat of which is taken from for example the rooms to be cooled. Disadvantages of this known system can be that the hot concentrated solution has to be continuously cooled what practically can result in energy losses and that no cooling can be obtained during the time period when there is no supply of heat. Thus, such a system cannot perform air-conditioning at night.
In U.S. Pat. No. 925,039 for William W. Seay a process of refrigeration is disclosed. Ammonia is in an absorber/generator tank absorbed by a solid salt, a thiocyanate of ammonium or of an alkali metal, to form a solution, the tank being cooled by cold water passing through a heat exchanger. In the absorbtion the dissolving of ammonia is an endothermal process, requiring or consuming energy, for which all of the latent heat of evaporation/condensation of the ammonia gas is used. This reduces the external cooling power required in the absorption process compared to processes in which the dissolving instead liberates energy, the latter case being advantageous when the process is also intended for heat generation. The solution is then heated by passing hot water through the heat exchanger. The ammonia is liberated from the solution, passes through another heat exchanger to be cooled thereby and condenses in a receiver tank. After most of the ammonia has condensed, valves are opened to let it expand and pass through a third heat exchanger or refrigerator element from which heat is drawn by the ammonia gas when it expands. The expanded gas then passes to the absorber/generator tank to start a new cycle.
It is an object of the invention to provide a chemical heat pump which can be driven by solar energy.
It is another object of the invention to provide a chemical heat pump in which advantages associated with a solid substance system are combined with advantages of a liquid substance system.
It is another object of the invention to provide a chemical heat pump in which efficient exchanging of heat between a liquid phase and a heat exchanger is obtained.
In a system using a solid substance which also after absorbing the volatile liquid remains solid a constant reaction pressure of the volatile liquid is maintained for a constant temperature of the substance when is absorbs vapor of the volatile liquid. The reaction pressure remains constant until all of the substance has been transferred from the first solid phase to the second solid phase. For a system, as suggested in the cited U.S. patent, which has a substance selected so that when in the discharging process vapor is absorbed by the substance the first phase is solid and the second phase is liquid, a solution phase, similarly a constant reaction pressure of the sorbate is maintained for a constant reaction temperature. The substance is then successively converted from a solid state to a liquid state. The process continues at a constant reaction pressure until all of the substance has been transferred to a liquid state. In the same way the reaction pressure in the last portion of the charging process is constant for a constant temperature when the substance is being converted from a liquid to a solid shape and vapor is liberated from the solution. In the first portion of the charging process the solution phase is only heated and no vapor is liberated. Thus, for such a heat pump using a phase transition between solid and liquid states advantages of a solid substance system can be combined with advantages of a liquid substance system.
When discharging the substance, i.e. when it absorbs the volatile liquid, the substance is made to be more and more dissolved in a somewhat diluted solution of the substance in the volatile liquid which exists in its vapor state around the solid and liquid phases. Thus, the produced solution is made to trickle over and through the remaining solid substance and is then passed through a filter or net to be separated from the solid substance. The solution which now becomes saturated then liberates heat produced both in the condensation of the vapor and in dissolving the vapor in the substance to a heat exchanger which is cooled by e.g. the outdoor air. This can be achieved by having a pump make the solution pass over a heat exchanger. The solution is then spread or distributed on some surface enlarging means in order to again participate in the absorbtion of vapor. The surface enlarging means can comprise balls, rods, nets, fibers made from some suitable material. The heat exchanger and the surface enlarging means can be combined in one unit.
Thus, in the process a three-phase system is used, in which vapor, a solid active substance and the saturated solution of the active substance simultaneously exist. In the discharging step these three components exist all the time. Thereby, for a constant temperature a constant vapor pressure is maintained. Thus, when starting the process, the major portion of the substance exists in its solid state. A minor portion exists in the saturated solution of the substance. In the discharging process the proportion between the amount of solid substance and the amount of solution is changed so that, at the end of the process, the major portion of the substance exists in the saturated solution. As long as a single crystal of the solid substance remains in the reactor, the three-phase rule is fulfilled and thereby the vapor pressure is constant for a constant temperature. Furthermore, the installation is designed to separate solid substance and saturated solution from each other before passing the pump and before passing the heat exchanging step. Thus, the exchanging of heat is performed completely in the liquid phase and thereby the exchanging of heat will be efficient. No melting of the substance is used. The efficiency of the process is determined by the capacity of the heat exchanger and the reaction between the saturated solution and the vapor which is turn depends on the size of the exposed area of the saturated solution and on the pressure drops in the system.
In the charging process, in the corresponding way, the three-phase system has to be considered. Both vapor, solid substance and saturated solution may exist simultaneously. The charging involves that the proportion of solid substance and saturated solution is changed towards more solid substance. When all three components or phases exist at the same time, in the same way as in discharging, the vapor pressure is constant provided that the temperature is constant. Saturated solution and particles of the solid substance are separated from each other by means of the net or filter and exchange of heat is made in the solution phase. The solution is distributed over a large area to liberate vapor of the volatile liquid.
Thus, the process in charging is completely reversible and uses the same basic setup as in discharging. However, it should be observed that in the charging process, owing to the higher temperature, the substance, which in the beginning of the charging exists almost completely in the solution phase, remains in the solution during part of the charging process since the solubility of the substances increases with temperature and then the system has only two phases. At some time in the charging process portions of the solution starts to be converted to solid substance and then the system again has three-phases. The different solubility at different temperatures means that in a way a small climb upwards on the temperature scale is made which obviously is necessary to increase the vapor pressure so that it is higher than in the condenser.
Making the saturated solution gradually crystallize and thereby obtaining a constant vapor pressure for a given temperature facilitates the charging process since it is then no longer necessary to increase the temperature of the reaction after a crystallization has started. Instead, if the substance, as in a normal absorption refrigeration process, had remained in a liquid state, the final charging temperature would have become as high as above 130xc2x0 C. for typical salt/water systems whereas, as used herein, it can be maintained well below 100xc2x0 C., often in the range of 70-85xc2x0 C.
In conventional absorption refrigeration systems, which for instance could employ a solution of LiBr, a crystallization must be avoided for process technical reasons. In such processes the volatile liquidxe2x80x94waterxe2x80x94in the solution can be liberated only if the temperature is allowed to rise highly above 100xc2x0 C. This is the basic problem encountered when obtaining refrigeration from solar driven cooling installations using absorption processes. The solution to this problem is to allow, during the step of increasing temperature, the water vapor to be freely emitted thereby leaving crystals of the solid substancexe2x80x94hydrate. When this occurs, i.e. when crystals start to be formed, the three-phase state is entered. The temperature then remains constant and at a significantly lower value than that of the corresponding absorption refrigeration process using a non-crystallizing solution (a liquid substance system). At this significantly lower temperature charging using solar energy is much more favorable and can be achieved without using costly, concentrating vacuum solar panels. In addition, an advantage is obtained by the solid substancexe2x80x94the hydrated saltxe2x80x94constituting a buffer which can be used for storing heat.
Similarly, it is quite possible to use, at the same time as the charging takes place, the saturated solution existing in the system for an absorption refrigeration process, i.e. refrigeration can be performed at day-time while the charging is made accumulating solid substance for future needs when the sun does not shine.
Generally the active substance must have the following property: It should at a first lower temperature have a solid state from which it, when absorbing the volatile liquid and in most cases the vapor phase thereof, directly passes partially to a liquid phase or a solution phase and at a second higher temperature it should have a liquid state or exist in a solution phase from which when emitting the volatile liquid which then is transferred to vapor it directly partially passes to a solid state. Preferred active substances working together with water vapor typically include various metal salts which in the solid state contain crystal water. Among these preferably magnesium chloride can be mentioned but also magnesium bromide and lithium chloride and some other salts can work well.
The vapor pressure of the volatile liquid should preferably be so low (which is equivalent to the condition that the quantity xcex94T defined below should be so large) that a sufficiently low cooling temperature is obtained. Further, the energy content in the reaction of the solid substance and the vapor should be sufficiently high in order to of interest in the intended applications, i.e. the solid substance should absorb a sufficient amount of water per final volume of the solution.
A solid substance being a mixture of salts can also be used, provided that it fulfils the requirements discussed above. An example of such a mixture is the dihydrate of calcium chloride mixed with a minor portion, say about 10% (weight), of lithium chloride.
In order to obtain a continuous operation of the heat pump, the various process steps can be made in separate spaces. Thus, a first space can be used for only charging the substance, i.e. heating a solution phase of the substance to produce solid substance and/or a saturated solution, a second space can be used only for condensing the vapor, a third space only for discharging, i.e. for making a saturated solution and solid substance absorb vapor, and a fourth space only for evaporating. Gas conduits are arranged so that the vapor can move freely between the first and second spaces and between the third and fourth spaces. Conduits and pumps are provided for transferring solution and volatile liquid between the first and second space and between the second and fourth spaces respectively.